(NaturalNews) It doesn't take a degree in nutrition to understand that eating fresh fruits and vegetables is an important part of a healthy lifestyle. But not all produce is created equal, and a significant portion of what is sold in grocery stores today as conventional produce has previously been treated with harsh chemicals, many of which end up lingering on, and sometimes in, the final product.

Levels of chemical residues on produce tend to vary dramatically, which is why the Environmental Working Group (EWG) releases a report every year spelling out the best and worst in each category. And the 2014 EWG Shopper's Guide to Pesticides in Produce is no exception, highlighting this year's "Dirty Dozen" and "Clean Fifteen" to help shoppers make better food-purchasing choices.

"EWG publishes its annual rating of conventional foods with the most and least pesticide residues to fill the void left by the U.S. Environmental Protection Agency, which has largely failed to tell Americans they have a right to know about the risks of pesticide exposure and ways they can reduce pesticides in their diets," explains the report.

Fruits and vegetables from every major category are selected and tested for pesticide and herbicide content, and EWG takes this data and compiles it into a user-friendly report that makes avoiding chemicals at the grocery store a lot simpler. This year, 48 unique produce items are included in the rankings, and some of the top most tainted might surprise you.

The Dirty Dozen

Topping the EWG Dirty Dozen list this year are conventional apples, which on average contained the highest concentrations of pesticides compared to any other produce item tested. An overwhelming 99 percent -- up from 98 percent in 2012 -- of apple samples were found to contain at least one pesticide residue, making organic or pesticide-free apples the only safe option.

Up a few spots this year are strawberries, which took second place on the Dirty Dozen list. In this latest analysis, conventional strawberries were found to contain 13 different pesticides, while earlier research by the U.S. Department of Agriculture (USDA) Pesticide Data Program identified as many as 54 different pesticide residues on conventional strawberries.

In the third, fourth, and fifth places are grapes, celery and peaches, respectively. A single grape tested was found to contain a shocking 15 different pesticides, while celery was found to contain 13 different pesticides. Peaches, due to their porous skin and soft flesh, were also discovered to retain high levels of multiple different pesticides.

The Clean Fifteen

The least important produce item to buy organic turned out to be avocados, a mere 1 percent of which showed any detectable levels of pesticides during tests. Not far behind them were sweet corn, pineapples, cabbage and sweet peas, each of which took the consecutive spots next to avocados.

Pineapples tested clean 89 percent of the time, according to the research, and the others were not far behind. No single fruit in the Clean Fifteen category tested positive for more than four types of pesticides, and mangos, kiwis, papayas and cantaloupes tested clean 88, 82, 80 and 61 percent of the time, respectively.

For the third year in a row, EWG also published a Dirty Dozen PLUS list, outlining foods that contain trace levels of extremely hazardous pesticides but that do not meet the traditional ranking criteria for the Dirty Dozen list. The two foods that made the PLUS list this year are leafy greens like kale and collard greens, and hot peppers, which often contain insecticides that damage the human nervous system.

To access the full EWG report on pesticides in produce, visit:EWG.org.